Three dimensional imaging has recently become of great importance. The problems encountered in three dimensional imaging are considerably more severe and complex than those presented in two dimensional imaging, due to the additional necessity to accurately control the exposed depth, and to the multilayered structure involved in the former case, since in most cases at least a portion of the underlying layer remains photoactive during the period that the overlying layers are being exposed to a different pattern, and vice versa.
Many systems for production of three-dimensional modeling by photoforming have been proposed. European patent application (Publication No. 250,121 filed by Scitex Corporation, Ltd. on June 6, 1987), and incorporated herein by reference, provides a good summary of documents pertinent to this art area, including various approaches attributed to Hull, Kodama, and Herbert. Additional background is described in U.S. Pat. No. 4,752,498 issued to Fudim on June 21, 1988, which is also incorporated herein by reference.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer.
One way of scanning is vector scanning, where only the areas corresponding to the shape of the rigid object have to be scanned, the amount of data to be stored is smaller, the data can be manipulated more easily, and "more than 90% of the CAD based machines generate and utilize vector data" (Lasers & Optronics, January 1989, Vol. 8, No. 1, pg. 56). The main reason why laser vector scanning has not been utilized extensively so far is the fact that, despite its advantages, it introduces problems related to the inertia of the optical members, such as mirrors, of the available deflection systems for the currently most convenient radiation sources, such as lasers. Since these systems are electromechanical in nature, there is a finite acceleration involved in reaching any beam velocity. This unavoidable non-uniformity in velocity results in unacceptable thickness variations in the exposed photoformable composition. Especially in the case of portions of layers having no immediate previous levels of exposure at the high intensity it becomes necessary to use high beam velocities, and therefore, longer acceleration times, which in turn result in thickness non-uniformity in the exposed composition.
Little attention has been paid so far to the composition itself by related art in the field of solid imaging, except in very general terms.
Thus, the compositions usually employed, present a number of different problems, major ones of which are shrinkage, warpage, brittleness, low overall cohesive strength, and most importantly overgrowth (i.e., layer hardening depthwise) of infraposed surfaces which are not immediately over solid substrate and adhesion between contiguous layers. Infraposed surface of a photoformed layer is the surface opposite to the directly exposed surface by the actinic radiation, and on the side of the liquid. Thus, it is difficult, if not impossible, to obtain both accurate image resolution in both width and depth and good adhesion between layers.
It is an object of this invention to resolve the above problems by using photoformable compositions comprising thermally coalescible materials, as explained in the detailed description cited below.
A major class of coalescible materials named plastisols are described in "Plastisols and Organosols", Edited by H. A. Sarvetnick, Published by Van Nostrand Reinhold Company, 1972.
Art relevant to photopolymerizable plastisol compositions and known to the applicant is cited below. Part of this art deals with polyvinyl chloride plastisols, part with acrylic and methacrylic plastisols, and part with different other classes of plastisols. None of this art, however, recognizes or deals with the problem of overgrowth of infraposed surfaces in partially photoformed multilayer structures, in which the unphotoformed materials have bee removed before thermal coalescence, or with the problem of adhesion between contiguous layers.
U.S. Pat. No. 4,634,562 describes a shapeable and photogellable liquid or fluid polyvinyl chloride composition comprising a blend of polyvinyl chloride with sufficient plasticizer and about 13 to 50 parts per hundred of a photopolymerizable polymer to give a liquid or fluid composition. Preferably said composition may contain a photoinitiator and a polyvinyl chloride stabilizer.
U.S. Pat. No. 4,623,558 describes an invention relating to a thermosetting plastisol dispersion composition comprising (1) poly(phenylene oxide) (PPO) in powder form, which is insoluble in the reactive plasticizer at room temperature and plasticizable at a temperature at or above the fluxing temperature; (2) a liquid reactive plasticizer member of the group consisting of (a) at least one epoxide resin having an average of more than one epoxide group in the molecule, (b) at least one liquid monomer, oligomer or prepolymer containing at least one ethylenically unsaturated group and (c) a mixture of (a) and (b); said liquid reactive plasticizer being capable of solvating the PPO at the fluxing temperature and being present in an amount ranging from 5 to 2,000 parts per 100 parts by weight of (1); and (3) 0.01 to 10% by weight of (2) of either a thermal initiator or photoinitiator for plasticizers present in the composition. Plastisol dispersion after fluxing can form a thermosetting after the crosslinking reaction.
U.S. Pat. Nos. 4,523,983 and 4,568,405 describe reactive plastisol dispersions comprising
(1) a polyvinyl acetal thermoplast in particle form;
(2) a liquid plasticizer;
(3) a free radical thermal initiator or
(4) a photoinitiator for crosslinking. The reactive plastisol dispersion when fluxed can be useful as a sealant or adhesive.
U.S. Pat. No. 3,615,448 describes a lithographic (planographic) printing plate prepared from an element comprising a layer of a photocurable composition containing finely divided particles of a vinyl plastic. The layer is exposed imagewise to U.V. radiation, which photocures the photocurable composition, and then is uniformly heat fluxed, whereby the photocurable composition plasticizes the plastic vinyl in the nonexposed areas. The imagewise exposure is through a stencil, or a negative or positive transparency (halftone or line). The exposed areas are either oleophilic or hydrophilic in relation to the nonexposed areas. The homopolymer of vinyl chloride is the preferred vinyl plastic.
U.S. Pat. No. 4,465,572 describes thermally coalescible acrylic resin plastisol or organosol dispersions of reduced viscosity and thixotropy result when 0.2 to 40% by volume of the total volume Of the dispersion of a volatile solvent is incorporated in compositions comprising particles of single-phase, random acrylic polymers or copolymers dispersed in nonvolatile plasticizers. Incorporation of a nonvolatile photopolymerizable component provides photosensitive dispersions useful for making relief and planographic printing plates, photoresists, and the like.
U.S. Pat. No. 4,4211,619 describes thermally coalescible acrylic resin dispersions comprising particles of single-phase, surfactant-free, random acrylic polymers or copolymers, containing at least 80% by weight of acrylic units, dispersed in a surfactant-free medium that comprises a compatible liquid plasticizer that is nonvolatile at room temperature and is not a monomer of any of the polymeric components. Incorporation of a photopolymerizable, ethylenically unsaturated compound provides photosensitive dispersions useful for making relief and planographic printing plates, photoresists, and the like.
U.S. Pat. No. 4,309,331 describes thermally coalescible acrylic resin dispersions comprising particles of single-phase, surfactant-free, random acrylic polymers or copolymers, containing at least 80% by weight of acrylic units, dispersed in a surfactant-free medium that comprises a compatible liquid plasticizer that is nonvolatile at room temperature and is not a monomer of any of the polymeric components. Incorporation of a photopolymerizable, ethylenically unsaturated compound provides photosensitive dispersions useful for making relief and planographic printing plates, photoresists, and the like.
U.S. Pat. No. 4,276,366 describes photoactive, thermally coalescible resin plastisol dispersions comprising a polyvinyl chloride polymer having attached to the polymer backbone photopolymerizable or photocrosslinkable groups dispersed in a liquid plasticizer medium, and a photoinitiator which is either a separate component or a photoactive substituent present on the polymer backbone. The photoactive plastisols, in element form, can be used in an image formation process that is a positive working washout or negative toned imaging system.
U.S. Pat. No. 4,251,618 describes photoactive, thermally coalescible resin plastisol dispersions comprising a polyvinyl chloride polymer having attached to the polymer backbone photopolymerizable or photocrosslinkable groups dispersed in a liquid plasticizer medium, and a photoinitiator which is either a separate component or a photoactive substituent present on the polymer backbone. The photoactive plastisols, in the element form, can be used in an image formation process that is positive working washout or negative toned imaging system.
U.S. Pat. No. 4,176,028 describes thermally coalescible plastisols and organosols of the invention comprise dispersions of single-phase particles of random polymers or copolymers that comprise organic polyelectrolytes that contain at least 1% by weight of ionizable monomer or comonomer, dispersed in a medium that comprises a nonvolatile liquid that is compatible with the resin. Incorporation of a photopolymerizable, ethylenically unsaturated compound provides photosensitive plastisols and organosols useful for making relief and planographic printing plates, photoresists, and the like.
U.S. Pat. No. 4,125,700 describes methyl methacrylate polymer powders especially suitable for making plastisols and organosols are prepared by an improved process comprising stepwise emulsion polymerization in aqueous media in the absence of emulsifiers and surfactants and in such manner that at least most of the monomer added at each step is consumed before the addition of the succeeding portion, followed by isolation of the polymer powder by evaporation of the latex at a temperature that is at least 30.degree. C., preferably 40.degree.-50.degree. C., below the glass transition temperature of the polymer.
Belgian Patent 865180 describes a thermally coalescible organosol or plastisol comprises a dispersion of an acrylic (co)polymer in a medium comprising (a) a compatible, non-volatile liquid which is not a solvent for a polymer and is not a monomer having the same chemical structure as that of the monomer from which the (co)polymer is derived, and (b) &lt;=40 vol. % (5-40 vol. %) based on total dispersion, of a volatile liquid which can dissolve or swell the polymer at room temp., pref. a chlorinated hydrocarbon, e.g., methylene chloride. The copolymer contains &gt;=50 wt. % acrylic units. The plastisols or organosols have low initial viscosity, low thixotropy and can be filtered. They can be made photosensitive by inclusion of a photopolymerizable monomer and the compositions obtained used e.g., to prepare relief and lithographic printing plates and photoresists.
In all cases of the related art discussed above, concerning thermally coalescible photoformable compositions, single coatings or single layers of such compositions are photoformed throughout their total depth in photoexposed regions, so that control of depth of photoforming is not critical. In contrast, when making multilayer three dimensional structures, control of the depth during photoforming the individual layers is extremely important for making models and prototypes (objects) with high resolution and high accuracy.